[doclint] Prepare doclint for more checks

This patch refactors doclint so that more checks and more generators
could be added.

This patch:
- Introduces 'Source' class, which holds file content in-memory and
  allows it to be updated.
- Introduces 'Message' class - which is a pair of a text and a type.
  Messages could have either 'error' type or 'warning' type.

package.json

@@ -11,7 +11,7 @@
     "unit": "jasmine test/test.js",
     "debug-unit": "DEBUG_TEST=true node --inspect-brk ./node_modules/.bin/jasmine test/test.js",
     "test-phantom": "python third_party/phantomjs/test/run-tests.py",
-    "test-doclint": "jasmine utils/doclint/test/test.js",
+    "test-doclint": "jasmine utils/doclint/check_public_api/test/test.js",
     "test": "npm run lint --silent && npm run coverage && npm run test-phantom && npm run test-doclint",
     "install": "node install.js",
     "lint": "([ \"$CI\" = true ] && eslint --quiet -f codeframe . || eslint .) && npm run doc",